1. Field of the Invention
The present invention relates to a transmission system wherein information carried by at least one analog signal and a data signal are multiplexed and transmitted. The present invention relates, in particular, to a multiplex transmission system wherein information carried by at least one analog signal including, for example, a voice signal and facsimile signal is multiplexed with a data signal and transmitted through an analog transmission (telephone) line of two wires or four wires.
2. Description of the Related Art
Recently, various types of signals including telephone (voice) signals, facsimile signals, data signals, and image data signals are multiplexed and transmitted in digital transmission systems. However, analog transmission lines are still mainly used in the presently equipped networks except basic transmission lines since it takes a long time for the digital transmission lines to prevail, and the running costs of the digital transmission lines are relatively high. Therefore, data signals are modulated by modems and transmitted through analog transmission lines, and then analog voice signals cannot be transmitted through the same analog transmission line as the analog transmission line through which the data signals modulated by modems are transmitted at the same time. Thus, development of a technique whereby various types of signals including telephone (voice) signals, facsimile signals, data signals, and image data signals can be multiplexed and transmitted in an analog transmission line, is required.
FIG. 1 is a diagram illustrating a transmission system connected between a main office 1000 and a branch office 1200, between the main office 1000 and a plurality of sub-branch offices 116-1 to 116-n, and between the branch office 1200 and a plurality of other sub-branch offices 116-1' to 116-n'. The main office 1000 and the branch office 1200 are connected by a digital transmission line 1400 through which digital multiplex signals are transmitted, where telephone (voice) signals, facsimile signals, data signals, and image data signals are multiplexed into the respective multiplex signals. However, taking the high running cost of the digital transmission lines into consideration, the main office 1000 and the plurality of subbranch offices 116-1' to 116-n are connected by a plurality of analog transmission lines 121-1 to 121-n and 122-1 to 122-n, where a plurality of data signals are modulated by modems 120-1 to 120-n and transmitted through the analog transmission lines 121-1 to 121-n, and the analog voice signals are transmitted through the analog transmission lines 122-1 to 122-n. Similarly, the branch office 1200 and the plurality of sub-branch offices 116-1' to 116-n' are connected by a plurality of analog transmission lines 121-1' to 121-n' and 122-1' to 122-n', where a plurality of data signals are modulated by modems 120-1' to 120-n' and transmitted through the analog transmission lines 121-1' to 121-n', and the analog voice signals are transmitted through the analog transmission lines 122-1' to 122-n'. Since transmission of data signals and transmission of analog signals are often carried out concurrently, the analog transmission lines 121-1 to 121-n and 122-1 to 122-n are separately used for transmitting the data signals and for transmitting the analog voice signals, respectively, between the main office 1000 and the plurality of sub-branch offices 116-1 to 116-n. Similarly, the analog transmission lines 1211' to 121-n' and 122-1' to 122-n' are separately used for transmitting the data signals and for transmitting the analog voice signals, respectively, between the branch office 1200 and the plurality of sub-branch offices 116-1' to 116-n'. The transmission system of FIG. 1 has a drawback that a great number of transmission lines must be subscribed to, and therefore, the cost for the subscription and equipment for the great number of transmission lines is great.